There have been conflicting reports about the effect of diabetes on bone density. In 1978, we studied 109 patients, 46 with type I and 63 with type II diabetes; ∼12 years later we restudied 35 of the 66 surviving patients. In the original study, radial bone density did not differ significantly between patients with either type of diabetes but was significantly lower than in nondiabetic control subjects. In eight osteopenic patients, bone formation rate and other histological indexes of osteoblast recruitment and function were markedly depressed compared with those in nondiabetic control subjects. In patients remeasured ∼2.5 years (41 patients) and ∼12.5 years (35 patients) after baseline, bone loss had continued at the expected rate in patients with type I diabetes, with maintenance of the same deficit, but was slower than expected in patients with type II diabetes, such that the initial deficit had been completely corrected. In six of the eight patients who had undergone bone biopsy, one with type I and five with type II diabetes, the mean bone mineral density z-score of the spine and femoral neck ∼12 years later was >0 and in one subject was significantly higher than normal at both sites. Based on these data and on previous studies, we propose that in patients with diabetes, low bone formation retards bone accumulation during growth, metabolic effects of poor glycemic control lead to increased bone resorption and bone loss in young adults, and low bone turnover retards agerelated bone loss. These effects account for low bone density in young patients with type I diabetes and normal or increased bone density in older patients with type II diabetes.